It comes from one of my favorite OC.
  1. Rein: derives from 'Rain' which refers to his tendency to interpret his surrounding in a gloomy way & hinting his mortal birthplace's climate. Also a naming custom.
  2. eon: an indefinite, or very long period of time. Refers to his proficiency in ancient arcanes & esoteric knowledge while additionally hinting his celestial age.
  3. Osborne: means 'godly/divinely being.'. Refers to his celestial background and full extent of his capability.

The only reason I did not include the middle name because I don't like how the name would look like if I try to add a 'space' by using ['] or similar symbols So I shorten it into 'Reiner' and deleting the 'eon', which would only make sense! Because the law of universe in world of FFXIV and the world he hailed from works differently!